完成评论功能

1. 定义评论的视图函数
   @app.route('/comment/',methods=['POST'])
   def comment()：
   读取前端页面数据，保存到数据库中
2. 用<input type="hidden" 方法获取前端的"question_id" 
3. 显示评论次数
4. 要求评论前登录
5. 尝试实现详情页面下的评论列表显示

   @app.route('/comment/', methods=['GET','POST'])
   @loginFirst
   def comment():
       comment = request.form.get('new_comment')
       ques_id = request.form.get('question_id')
       auth_id = User.query.filter(User.username == session.get('user')).first().id
       comm = Comment(author_id = auth_id, question_id = ques_id, detail=comment)
       db.session.add(comm)
       db.session.commit()
       return redirect(url_for('detail'))

   {% extends 'base.html' %}
   {% block title %}问答详情{% endblock %}
   {% block main %}
   <div class="page-header">
   <h3>题目：{{ ques.title }}<br><small>作者：{{ ques.author }} 发布时间：{{ ques.create_time }}</small></h3>
   </div>
   <p class="lead">detail{{ ques.detail }}</p>
   <hr>
   <form action="{{ url_for('question') }}" method="post">
   <div class="form-group">
   <textarea name="new_comment" class="form-control" rows="3" id="new_comment" placeholder="write your comment"></textarea>
       <input type="hidden" name="questions_id" value="{{ questions.id }}">
   </div>
   <button type="submit" class="btn btn-default">发送</button>
   <h4>评论({{ questions.comments|length }})</h4>
   </form>
   <ul class="list-group" style="margin: 10px"></ul>
   
   {% endblock %}

   def loginFirst(func):
       @wraps(func)
       def wrapper(*args,**kwargs):
           if session.get('user'):
               return func(*args,**kwargs)
           else:
               return redirect(url_for('login'))
       return wrapper

   <ul class="list-group">
   {% for foo in questions %}
       <li class="list-group-item">
           <span class="glyphicon glyphicon-leaf" aria-hidden="true"></span>
           <a href="#">{{ foo.author.username }}</a>
           <br>
           <a href="{{ url_for('detail',question_id=foo.id) }}">{{ foo.title }}</a>
           <span class="badge">{{ foo.creat_time }}</span>
           <p style="color:black">{{ foo.detail }}</p>
       </li>
   {% endfor %}
   </ul>